The flexibility and reliability of communication networks based upon transmissions of light signals via optical fibers are greatly increased by the availability of assemblies such as optical circulators and isolators. For example, a three-port circulator may be used to enable a single fiber to be used for bidirectional communications between two remote sites. By utilizing non-reciprocal optical elements, i.e. elements which affect light moving in different directions differently, a bidirectional fiber may be optically coupled to both an input fiber and an output fiber. Non-reciprocal operations provide differences in the "walk-off," i.e. spatial displacement, of oppositely directed light beams, so that the input and output fibers are optically isolated from each other.
An optical isolator may include only single-mode fibers. The input fiber directs light signals into an optical assembly that splits the light into polarization components, performs non-reciprocal operations on the components, and recombines the components for output at the output fiber. The non-reciprocal operations are designed to reduce the likelihood that back-directed light will be aligned with the input fiber.
U.S. Pat. No. 4,464,022 to Emkey describes an optical circulator that includes a first birefringent plate that is used to separate a beam from a first port into two beams having orthogonal polarizations. The two beams are then recombined by a second birefringent plate positioned at a second port. Between the first and second birefringent plates is a Faraday rotator that provides non-reciprocal rotation of the polarizations. A third birefringent plate and a reflecting element are positioned between the third and second birefringent plates, so that the beams are reflected at a ninety degree angle to the second port. However, the reflecting element has a slotted portion that allows light from a third port to pass through the reflecting element toward the first port. Thus, the first port acts as an input port for transmission of signals via the second port, but acts as an output port for signals from the third port. The ninety degree angle of the second port relative to the first and third ports is designed to maximize the isolation of the second port from the third port. A similar arrangement is described in U.S. Pat. No. 5,212,586 to Van Delden.
There are a number of factors that must be considered in the design of optical circulators and isolators. U.S. Pat. No. 5,319,483 to Krasinski et al. identifies insertion loss and crosstalk as two performance-related design considerations. Insertion loss is the difference in power between input light and the light that exits the optical assembly. The primary causes of insertion loss are identified as absorption of light and imperfections of polarization separation and recombination. Crosstalk in an optical circulator is the transmission of light from an input fiber to a fiber which is not the intended output fiber. Krasinski et al. assert that the primary cause of crosstalk in optical circulators is back-reflection from the various optical elements in the assembly. The system described in the patent utilizes birefringent crystals instead of polarization reflecting elements and polarization splitting cubes in an attempt to provide more complete polarization separation, thereby reducing insertion loss and crosstalk. Moreover, the system is one in which the optical circulators of the assembly are in optical contact with each other, thereby reducing back-reflections.
While known optical circulators and isolators operate well for their intended purposes, further improvements in performance are desired. Another design goal is to increase cost efficiency in the fabrication of optical circulators and isolators.
What is needed is an optical assembly and method for efficiently transferring optical signals within a multi-port system, such that each port is selectively coupled with respect to exchanges of signals among the remaining ports. What is also needed is such an optical assembly that is fabricated cost efficiently.